1. Field of the Invention
The present invention relates to multicast routing programs, multicast routing methods, and multicast routers for routing multicast data. In particular, the present invention relates to a multicast routing program, a multicast routing method, and a multicast router for allowing multicast data to be transmitted over a transmission line different from a transmission line for unicast data.
2. Description of the Related Art
Recent development of the advanced information society has promoted the proliferation of optical fibers all over the nation. With this widespread use of optical fibers, the Internet Protocol (IP) network communication speed has also been increased. These higher communication speeds have brought about new forms of communication based on IP multicast, such as moving-image distribution services and network game applications, in addition to conventional forms of communication including World Wide Web and electronic mail applications.
Several types of IP multicast technologies are currently available. Among others, Protocol Independent Multicast-Sparse Mode (PIM-SM) for efficiently using network traffic is a typical IP multicast technology, which is employed in many IP multicast networks.
FIG. 18 is a diagram depicting a technology for transmitting multicast data by known dynamic routing. A plurality of optical fibers is laid down among routers 901 to 906 (including layer 3 switches). In other words, the routers 901 to 906 are interconnected via a plurality of paths. Connecting the routers via a plurality of paths allows an alternative route to be automatically selected by dynamic routing in the event of a line failure such as a breakage of an optical fiber.
Network segments 910 and 920 are connected to the router 901. A terminal device 911 is connected to the segment 910. A terminal device 921 is connected to the segment 920.
Network segments 930 and 940 are connected to the router 906. A terminal device 931 and a multicast-transmission terminal device 932 are connected to the segment 930. A terminal device 941 and a multicast-transmission terminal device 942 are connected to the segment 940. The multicast-transmission terminal devices 932 and 942 generate and output source data to be multicast.
This network controls data paths by Open Shortest Path First (OSPF). The routers 901 to 906 each acquire segment information by dynamic routing and generate a unicast routing table. The unicast routing tables include a cost of each path among the routers 901 to 906.
A path passing through the routers 901, 902, 904, and 906 has a cost of “1” between adjacent routers. A path passing through the routers 901, 903, 905, and 906 has a cost of “10” between adjacent routers. The routers 901 to 906 carry out data routing by selecting a path with a lower cost.
It is assumed that unicast communication is carried out between the terminal device 911 and the terminal device 931. Unicast data 951 is transmitted and received through a path with a lower cost. Therefore, the unicast data 951 is transmitted and received via the path over the routers 901, 902, 904, and 906.
A procedure for transmitting multicast data by the PIM-SM technology will be described next. This example assumes that multicast data 953 is distributed from the multicast-transmission terminal device 932 to the terminal device 911.
To enable the terminal device 911 to receive the multicast data 953 from the multicast-transmission terminal device 932, the terminal device 911 transmits to the router 901 a multicast reception request (join message) 952 bound for the unicast address of the multicast-transmission terminal device 932. The multicast reception request 952 is periodically transmitted by the terminal device 911.
When the router 901 has received the multicast reception request 952, it refers to the unicast routing table to select the path with the lowest cost from among the paths connected to the multicast-transmission terminal device 932 and then transmits the multicast reception request 952 to the adjacent router 902 on the selected path.
This operation of transmitting the multicast reception request 952 is sequentially repeated at the routers on the path up to the router 906 connected to the segment 930, including the multicast-transmission terminal device 932. When the router 906 receives the multicast reception request 952, it outputs the multicast data 953 through the communication interface that has received the multicast reception request 952. The routers 904, 902, and 901 also transfer the multicast data 953. As a result, the multicast data 953 is transmitted to the terminal device 911, which has transmitted the multicast reception request 952.
When the multicast data 953 is distributed by the PIM-SM technology in this manner, the multicast data 953 output from the multicast-transmission terminal device 932 in the segment 930 and the unicast data 951 output from the terminal device 931 in the segment 930 are transmitted to the terminal device 911 over the same path.
In dynamic routing, a data reception path is automatically switched in the event of a line failure or other problems. Also in this case, multicast data and unicast data are transferred along the same path.
Instead of transmitting a multicast reception request along unicast routing, a destination to which the multicast reception request is transmitted can be statically preset.
FIG. 19 is a diagram depicting a technology for transmitting multicast data by known static routing. In a network shown in FIG. 19, the routers 901 to 906 on the network shown in FIG. 18 are replaced with routers 961 to 966, respectively, for performing static routing. The transfer of unicast data 971 is carried out according to the unicast routing tables in the same manner as in FIG. 18.
A multicast reception request 972 (join message) is statically preset in the routers 961 to 966. In the case of FIG. 19, in the router 961, the transfer destination of the multicast reception request 972 specifying the multicast address of the multicast-transmission terminal device 932 is set to the router 963. Similarly, in the router 963, the transfer destination of the multicast reception request 972 specifying the multicast address of the multicast-transmission terminal device 932 is set to the router 965. In the router 965, the transfer destination of the multicast reception request 972 specifying the multicast address of the multicast-transmission terminal device 932 is set to the router 966.
When the multicast reception request 972 specifying the multicast address of the multicast-transmission terminal device 932 is output from the terminal device 911, the multicast reception request 972 is passed to the router 961. The router 961 transmits the multicast reception request 972 to the router 963 according to the static setting. At this time, no reference is made to the unicast routing table. Subsequently, the multicast reception request 972 is transferred from the router 963 to the router 965 and further from the router 965 to the router 966.
When the router 966, which has received multicast data 973 from the multicast-transmission terminal device 932, receives the multicast reception request 972 from the router 965, the router 966 transmits the multicast data 973 to the line over which the multicast reception request 972 has been received. When the routers 965, 963, and 961 receive the multicast data 973, they also transmit the multicast data 973 to the line over which the multicast reception request 972 has been received.
Consequently, the multicast data 973 is transmitted to the terminal device 911. The multicast reception request 972 is periodically transmitted from the terminal device 911.
A network relay device for selecting an optimal path according to the type of transmission data and forwarding reception data to the selected path is also disclosed (for example, see Japanese Unexamined Patent Publication No. 2003-78556).
However, multicast data transferred by dynamic routing always goes over the same path as that for unicast data. More specifically, when a router transfers a multicast reception request output from a terminal device, the router searches the unicast routing table for the unicast address of the multicast-transmission terminal device and transmits the multicast reception request through the shortest path. This transmission path for the multicast reception request is the same as that for unicast data. The multicast data goes in the reverse direction over the path taken by the multicast reception request. This means that the multicast data and the unicast data always use the same line.
For this reason, multicast data and unicast data cannot be distributed via different routes.
On the other hand, static multicast routing allows multicast data and unicast data to be distributed via different routes. In this case, however, the multicast data cannot be automatically diverted to the line for unicast data in the event of a failure on the line set for static multicast routing. Therefore, network reliability deteriorates.
Japanese Unexamined Patent Publication No. 2003-78556 also discloses a technology for carrying out path selection according to the protocol of communication data. In this technology, however, the transmission line for multicast data is determined depending on the transmission line taken by a multicast reception request output from a terminal device. Therefore, once the multicast reception request has been transmitted over the same path as that for unicast data, it is difficult to transmit multicast data output from the multicast-transmission terminal device via a route different from that for the unicast data even if the path selection technology described in Japanese Unexamined Patent Publication No. 2003-78556 is applied to the multicast data.
Multicast data can be prevented from being output to irrelevant routes by transferring the multicast data in the reverse direction over the path taken by the multicast reception request. This processing is necessary to make communication efficient. Thus, if a technology disclosed in Japanese Unexamined Patent Publication No. 2003-78556 is used, regardless of the path taken by the multicast reception request, to distribute unicast data and multicast data via different routes, the multicast data is transmitted to irrelevant routes. This means that it is difficult to apply the technology, described in Japanese Unexamined Patent Publication No. 2003-78556, directly to unicast data and multicast data to be distributed via different routes.